Question: $ D = \left[\begin{array}{rr}-1 & 1 \\ 1 & -2\end{array}\right]$ $ w = \left[\begin{array}{r}-1 \\ 4\end{array}\right]$ What is $ D w$ ?
Answer: Because $ D$ has dimensions $(2\times2)$ and $ w$ has dimensions $(2\times1)$ , the answer matrix will have dimensions $(2\times1)$ $ D w = \left[\begin{array}{rr}{-1} & {1} \\ {1} & {-2}\end{array}\right] \left[\begin{array}{r}{-1} \\ {4}\end{array}\right] = \left[\begin{array}{r}? \\ ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ w$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ w$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ w$ , and so on. Add the products together. $ \left[\begin{array}{r}{-1}\cdot{-1}+{1}\cdot{4} \\ ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ w$ and add the products together. $ \left[\begin{array}{r}{-1}\cdot{-1}+{1}\cdot{4} \\ {1}\cdot{-1}+{-2}\cdot{4}\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{r}{-1}\cdot{-1}+{1}\cdot{4} \\ {1}\cdot{-1}+{-2}\cdot{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{r}5 \\ -9\end{array}\right] $